(1) Field of the Invention:
The present invention relates to an electrophotographic photosensitive material and more particularly to an electrophotographic photosensitive material which is devised to permit the production of a high quality image by incorporating a specified organic compound in a surface protective layer.
(2) Description of the Prior Art:
Various electrophotographic photosensitive materials have hithereto been well known in the electrophotographic art, for instance, the so-called binder type electrophotographic photosensitive material which comprises coating the surface of a conductive support with a solution obtained by dispersing an inorganic photoconductive substance such as zinc oxide, titanium oxide or the like in a binder (binder resin), and an electrophotographic photosensitive material which comprises depositing on a conductive support an organic photoconductive substance such as poly-N-vinyl carbazole or the like.
These electrophotographic photosensitive materials (which hereinafter will be called "photosensitive material" in short) have at present been considerably improved to such an extent that they can produce high quality copies when developed using a toner composed mainly of a coloring pigment such as carbon black or the like or a magnetic substance and a resin.
By the way, the aforesaid photosensitive materials include a type which comprises further forming a protective layer on a photocondutive layer. This protective layer is provided mainly for the purpose of protecting the photoconductive layer. However, the protective layer-provided photosensitive material of this sort has a tendency that the background is easily stained and copies of poor sensitivity are liable to be produced. Presumedly, this takes place due to the electrically insulating property of the protective layer.
It is of course known as a solution to this problem to add a substance having a superior ability to accept an electric charge to a protective layer for the purpose of preventing the occurrence of residual charge. As the additives used herein there can be enumerated carbon black, metal powder, tetra-ammonium salt and so forth.
However, when these additives are contained in a protective layer, it brings about the effect of promoting the conductivity of the protective layer and thus preventing the accumulation of electric charge, whilst it entails a disadvantageous effect on the protective layer, that is, the ability to retain an electric charge in the dark is deteriorated, said ability being an indispensable requisite in the field of electrophotography. It is, in effect, extremely difficult to find an amount of the additive which can prevent the accumulation of electric charge as well as promote the ability to retain an electric charge in the dark simultaneously.
Furthermore, Japanese Laid-open Patent Application No. 3992/1975 (the basic application P-2336094.3 was filed in West Germany by Hoechst Company) discloses a recording material for use in electrophotography which comprises forming on an amorphous selenic photoconductive layer an overcoating layer containing a dialkylamino radical-substituted heterocyclic electron donor and if needed, a sensitizer, i.e., a dyestuff of any kind, thereby promoting the transfer of the electric charge generated on said photoconductive layer. However, when reproduction is repeated using this recording material, it gives rise to a residual potential owing to light-fatigue, thereby hampering the production of satisfactory images.